We have developed a rapid, quantitative assay for the measurement of an activator of plasminogen in blood. Plasminogen when activated is a potent fibrinolytic (clot-dissolving) enzyme. Our assay also enables us to measure a precursor of the actiator (proactivator). This assay will be used to isolate components of the blood system and to study compounds and procedures which affect the blood fibrinolytic levels. A fragment of streptokinase (SK), M.W. 25,700, obtained after SK has reacted with dog plasminogen is biologically inactive. A similar fragment, M.W. 33,000, obtained after SK reacts with human plasminogen is biologically active. These fragments will be isolated and compared to determine the specific molecular structure required for biological activity. After depletion of plasminogen in the cat, a new type of plasminogen has been isolated which is different from normal, circulating plasminogen. This finding raises new posibilities concerning plasminogen metabolism, detection and effectiveness as a therapeutic agent, which will be explored. The possible role of plasmin in cell differentiation as described by Reich and collaborators will be investigated with depleted and enriched growth media. BIBLIOGRAPHIC REFERENCES: Requirement of Human Plasmin for Streptokinase Activation of Bovine Plasminogen. K.N.N. Reddy and D.L. Kline, Thrombosis Research 6: 481-488, 1975.